A variety of electronic circuits are used to drive diode loads and, more particularly, to control electrical current through strings of series-connected light-emitting diodes (LEDs), which, in some examples, form an LED display, or, more particularly, a backlight for a display, for example, a liquid crystal display (LCD). It is known that individual LEDs have a variation in forward voltage drop from unit to unit. Therefore, the strings of series-connected LEDs can have a variation in forward voltage drop.
Strings of series-connected LEDs can be coupled to a common switching regulator, e.g., a boost switching regulator, at one end of the LED strings, the switching regulator configured to provide a high enough voltage to supply each of the strings of LEDs. The other end of each of the strings of series-connected LEDs can be coupled to a respective current sink, configured to sink a relatively constant current through each of the strings of series-connected LEDs.
It will be appreciated that the voltage generated by the common switching regulator must be a high enough voltage to supply the one series-connected string of LEDs having the greatest total voltage drop, plus an overhead voltage needed for proper operation of the respective current sink. In other words, if three series-connected strings of LEDs have voltage drops of 23 Volts, 21 Volts and 20 Volts, and each respective current sink requires at least 0.6 Volts in order to operate, then the common boost switching regulator must supply at least 23.6 Volts.